Never scale navbar scrim in all apps and widget picker

The navbar scrim is scaled during the predictive back animation. This should be prevented since the navbar itself does not change it's position or scale.

Bug: 373544911
Test: Manual, i.e. verifying that the navbar scrim remains in its original position during the predictive back animation (with 3-button-nav) on phones and tablets
Flag: com.android.window.flags.predictive_back_three_button_nav
Change-Id: Iaffd31de5493dea8c856ad3d5b441cca066bbad8